1. Field of the Invention
The present invention is directed to a method for the operation of a magnetic resonance tomography apparatus of the type having a basic field magnet, a gradient system with gradient coils and a control system that, among other things, controls the currents in the gradient coils to produce pulse sequences, as well as directed to an apparatus for the implementation of the method and to a method for designing a magnetic resonance tomography apparatus.
2. Description of the Prior Art
Magnetic resonance tomography is a known modality for acquiring images of the inside of the body of a living patient. The basic components of a magnetic resonance tomography apparatus are a basic field magnet, a gradient system and a control system that, to produce pulse sequences on the basis of pulse sequences, controls the currents in the gradient coils to produce pulse sequences. The time-variable coil currents achieve amplitude values of up to several 100 A and are subject to frequent and rapid changes in the direction of the current with rise and decay rates of several 100 kA/s. Given the presence of a basic magnetic field, these currents in the gradient coils cause vibrations due to Lorentz forces, which cause noise by vibrating components of the apparatus.
The focus of previous investigations for reducing noise has been to modify the transmission path, i.e. to a modify the mechanical structure of the magnetic resonance tomography apparatus. These modifications were roughly implemented on the basis of empirical values. Further, noise that differs in degree is only heard by an operator as a patient as a result of various pulse sequences after the implementation of the pulse sequences, as a subjective impression. Developments in the field of magnetic resonance tomography for shortening the measuring time and improving the image quality involve a boost of the currents in the gradient coils. The noise thus also increases.
In the article by R. A. Hedeen et al., xe2x80x9cCharacterization and Prediction of Gradient Acoustic Noise in MR Imagersxe2x80x9d, Magnetic Resonance in Medicine 37 (1997), pages 7-10, a noise that a pulse sequence causes when executed is calculated in advance before the execution of the pulse sequence, a transfer function is empirically derived from gradient pulses as an input quantity and noise as an output quantity. This transfer function is multiplied by the spectrum of the gradient pulses to be implemented. A noise spectrum identified in this way is substantially similar to the spectrum of the gradient pulses and is only slightly influences by acoustic resonances of the apparatus. Measures for direct reduction of the noise are not described in this article.
An object of the present invention is to provide a method or and an apparatus for the implementation of the method for obtaining information about the noise stress on a patient as well as for reducing noise, and to provide a method for the design of a gradient system that enables an optimization of the overall noise transmission path.
This object is achieved in a method wherein noise that a pulse sequence causes when implemented is determined before the start of the pulse sequence, and wherein, in the case of identified noise above a selectable value, the pulse sequence is modified, so that the modified pulse sequence does not exceed the selectable value when implemented.
The apparatus for the implementation of the method has a unit that determines noise that a pulse sequence causes when implemented, before a start of the pulse sequence, and a display that displays the identified noise, and where a control unit, given a modification of the displayed noise, modifies the noise-relevant parameters of the pulse sequence such that the modified display value of the noise is not exceeded upon implementation of the pulse sequence with the modified parameters. Alternatively the apparatus determines noise that a pulse sequence causes upon implementation thereof before a start of the pulse sequence, and checks the identified noise of the pulse sequence to determine whether a pre-select able value is exceeded, and determines a modification of the pulse sequence when this value is exceeded so that the modified pulse sequence does not exceed the pre-select able value when implemented, which causes and the modified pulse sequence to be implemented.
The object of providing a method for designing a gradient system is inventively achieved by deriving a transfer function from a gradient coil current as an input quantity and noise as output quantity, this transfer function being calculated dependent on a geometry, on material parameters and on a mechanical implementation of the apparatus.
The inventive method and apparatus have the advantage that instead of the noise for different pulse sequences being present only after the implementation of a pulse sequence and as a subjective impression or as a result of a measurement, the noise is calculated before the implementation of a pulse sequence and, following thereupon, a noise reduction becomes possible by simple modification of parameters of the pulse sequence.
In an embodiment, a repetition time of the pulse sequence is modified, when a modification of the pulse sequence is needed. A modification of the repetition time is a simple modification of a pulse sequence. The modification needed for noise reduction is simple to calculate given unaltered demands on the image quality.
In an another embodiment, the repetition time is modified so as to avoid a coincidence of whole-number multiples of a reciprocal of the repetition time with acoustic resonant frequencies of the apparatus.
In another embodiment in the pulse sequence modification, a forbidden frequency band of, preferably, xc2x120 Hz of the resonant frequency is defined around at least one of the resonant frequencies and any modification which would result in a frequency in this forbidden band is precluded.
In a further embodiment, the determination of the noise is implemented in the frequency domain, by deriving a system-inherent transfer function that from a gradient coil current as an input quantity and the noise as an output quantity, and this transfer function is multiplied, for each gradient coil, by the Fourier transform of the coil current for that gradient coil, the multiplication results are integrated over the frequency, and the integration results along the individual gradient axes are summed. Only transformation in the frequency domain enables a separation of the parameters and an exact calculation of the influences on the noise that are caused even by minute modifications of the parameters.
In another embodiment, the A acoustic pressure level of a pulse sequence is determined for this purpose by an A-weighted integration of the multiplication results. A weighting quantity is thus available for the noise of a pulse sequence that takes the properties of human hearing into consideration.
In another embodiment, one of the transfer functions is determined by excitation of the corresponding gradient coil with a current whose spectrum contains all noise relevant frequencies. All relevant frequency components of the transfer function thus are measured simultaneously with a single excitation. This results in the shortest measuring times in the experimental determination of the transfer functions.
In another embodiment, one of the transfer functions is determined by excitation of the corresponding gradient coil with a frequency sweep of sinusoidal currents. As a result, a discrete frequency point of the transfer function is determined with a defined amplitude with each sine oscillation.
In another embodiment, the Fourier analysis of the gradient coil current is implemented with a reciprocal of the repetition time of the pulse sequence as the fundamental frequency of the Fourier analysis. The Fourier analysis thus exclusively yields Fourier coefficients for whole-numbered multiples of the reciprocal of the repetition time.
In a further embodiment, the identified noise is displayed. An operator, for example, thus can evaluate whether a patient can reasonably withstand a noise stress dependent on condition and age, and can take this in the operator""s other operating actions.
In another embodiment, an automatic modification of the pulse sequence to lower noise values is implemented when the identified noise exceeds a pre-selectable value. As a result, the operator is relieved of the need to make decisions on a case-by-case basis about the reasonableness of the noise, and that the modification of the pulse sequence toward lower noise values ensues automatically dependent on the pre-select able value without operator intervention.